1. Field of the Invention
The present invention relates to an optical disk used for the recording and reproduction of information, and a method for manufacturing the same.
2. Description of Related Art
In the field of optical disks, there is a demand for information to be recorded at ever higher densities. To achieve higher density recordings, it has been proposed to record optical disks by DWDD (domain wall displacement detection), which is a type of super-resolution technology.
In DWDD optical disks the magnetic coupling between adjacent recording tracks must be weakened (reduction of magnetic anisotropy). Thus, in the manufacture of DWDD optical disks, an initialization (hereinafter referred to as “annealing”, “annealing method”, or “annealing process”) is performed to weaken the magnetic coupling between adjacent recording tracks before information signals are recorded. A method for such annealing has been reported in the prior art (see JP-H06(1994)-290496A and JP-H10(1998)-340493A).
An example of the structure of a conventional optical disk and annealing method for the same is shown in FIG. 9. As FIG. 9 shows, a conventional optical disk 1 is provided with a substrate 2, and a first dielectric layer 3, a recording layer 4, a second dielectric layer 5, and a protective coat layer 6 sequentially layered on the substrate 2. Grooves 2a are formed in the surface of the substrate 2 on the recording layer 4 side. The area between two radially adjacent grooves 2a is called a land, and serves as the recording track. The width of the grooves 2a is 0.2 μm, for example, and the width of the land portion is 1.4 μm. The recording layer 4 is provided with three or more magnetic layers for reproduction by DWDD.
The method of annealing the optical disk 1 will be described next. In the optical disk 1, the magnetic coupling of the recording layer 4 above the grooves 2a is erased by irradiating laser light 7 (laser power: 10 mW, λ=780 nm, objective lens 8 of NA=0.5, diameter of laser spot: approximately 800 nm) for the annealing along the grooves 2a. In this annealing process, the movement speed of the laser spot of the laser light 7 is for example 2 m/sec.
In the above annealing method, however, sections other than the grooves 2a are also irradiated by the laser spot, which leads to a narrower effective recording track and causes the signal level to drop. For this reason, the laser spot irradiated on the recording layer 4 must be made smaller, but because the first dielectric layer 3 is optimized to the wavelength of the recording/reproducing laser light, it has been difficult to reduce the size of the laser spot of the annealing laser light 7. That is to say, attempting to maintain the effective track width while also securing the annealing width meant that the track pitch could not be made smaller and higher densities could not be achieved.
For the same reason, since it was difficult to increase the absorption efficiency of the recording layer 4 with respect to the annealing laser light 7, there was the problem that the annealing could not be performed at high linear speeds, so that the annealing was time-consuming. Moreover, fluctuations in the annealing power lead to variations in the annealing width, thereby causing variations in the effective track width and making it difficult to obtain good recording/reproducing properties. Furthermore, the annealing power at which an annealing of a specific width can be achieved varies disk by disk or varies with the radial position of the optical disk depending on the process, so that the track pitch must be determined with consideration to these variations, which stands in the way of higher densities.